<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .item {
            float: left;
            width: 280px;
            height: 120px;
            border: 1px solid red;
        }
    </style>
</head>
<body>
    <div style="background: blue">
        <div class="item">左边</div>
        <div class="item">左边</div>
        <div class="item">左边</div>
        <div class="item">左边</div>
        <div style="float: right">右边</div>
        <div style="clear: both"></div>
    </div>
    <div>你好呀</div>

</body>
</html>